How to seal the gutter joint

What materials to use? How to seal the gutter joint made of galvanized steel? Soldering is not an option, because the gutters are old. The reason for the repair is this, that the gutter is approx 50 m and only one trigger, and it is difficult to convince the owners of other garages to replace the entire garage. I am asking for help in the implementation of the seal in the form of technical advice, what materials to use, to be successful.

Emergency repair is best done with silicone (when it comes to sealing the connection only) or Subfield (when you need to patch the hole with a piece of sheet metal). For the future, however, you have to think about dividing the common gutter into shorter sections with individual drains or abandon it altogether, because the tray is long on 50 m is little better than nothing.